1. Field of the Invention
The present invention relates to a solid electrolyte fuel cell configuration, more particularly relates to a solid electrolyte fuel cell configuration comprised of a solid electrolyte substrate formed with pluralities of anode layers and cathode layers and enabling a smaller size, greater thinness, and higher output by a simple structure not requiring sealing.
2. Description of the Related Art
In the past, fuel cell configurations have been developed and put into practical use as low polluting power generating means for taking the place of thermal power generation or as sources of electrical energy for electric cars for taking the place of engines fueled by gasoline etc. Considerable research is going on for increasing the efficiency and reducing the cost of such fuel cell configurations.
These fuel cell configurations generate power by various systems. Among these, there are types of fuel cell configurations using solid electrolytes. As one example of a fuel cell configuration using a solid electrolyte, there is one using a sintered body comprised of yttria-(Y2O3) stabilized zirconia as an oxygen ion transfer type solid electrolyte layer. One side of this solid electrolyte layer is formed with cathode layers, while the other side is formed with anode layers. Oxygen or an oxygen-containing gas is supplied to the anode layer side, while methane or another fuel gas is supplied to the anode layer.
In the fuel cell configuration, the oxygen (O2) supplied to the cathode layers is ionized to oxygen ions (O2−) at the boundary between the cathode layers and solid electrolyte layer. The oxygen ions are transferred to the anode layers by the solid electrolyte layer and supplied to the anode layers. For example, they react with the methane (CH4) gas, whereby finally water (H2O) and carbon dioxide (CO2) are produced. In this reaction, the oxygen ions release electrons, so a potential difference occurs between the cathode layers and the anode layers. Therefore, if attaching lead wires to the cathode layers and anode layers, the electrons of the anode layers flow through the lead wires to the cathode layer side resulting in the generation of power in the fuel cell configuration. Note that the drive temperature of this fuel cell configuration is about 1000° C.
However, in this type of fuel cell configuration, it is necessary to provide separate chambers comprised of an oxygen or oxygen-containing gas supply chamber at the cathode layer side and a fuel gas supply chamber at the anode layer. Since the layers are exposed to an oxidizing atmosphere and a reducing atmosphere under a high temperature, it is difficult to improve the durability of the fuel cells.
On the other hand, a fuel cell configuration has been developed comprised of a solid electrolyte layer provided at opposite sides with cathode layers and anode layers to form fuel cells placed in fuel gas, for example, mixed fuel gas comprised of methane gas and oxygen gas mixed together, so as to generate an electromotive force between the cathode layers and anode layers. In this type of fuel cell configuration, the principle of generation of the electromotive force between the cathode layers and the anode layers is similar to the case of the above fuel cell configuration of the separate chamber type, but it is possible to place the fuel cells as a whole in substantially the same atmosphere, so it is possible to use a single chamber in which a mixed fuel gas is supplied and possible to improve the durability of the fuel cells.
However, even in this single chamber type fuel cell configuration, the configuration has to be driven at a high temperature of about 1000° C., so there is the danger of explosion of the mixed fuel gas. To avoid this danger, if making the oxygen concentration a concentration lower than the ignition limit, the problem arises that carbonization of the methane or other fuel proceeds and the cell configuration performance drops. Therefore, a single chamber type fuel cell configuration able to use a mixed gas of a concentration of oxygen able to prevent the progress of carbonization of the fuel while preventing explosion of mixed fuel gas has been proposed (for example, see Japanese Unexamined Patent Publication (Kokai) No. 2003-92124).
The configuration of the proposed single chamber type fuel cell configuration is shown in FIG. 12A. This fuel cell configuration is structured by fuel cells including solid electrolyte layers stacked in parallel to the flow of the mixed fuel gas. The fuel cells are comprised of dense structure solid electrolyte layers 1 and porous cathode layers 2 and anode layers 3 formed at the two sides of the solid electrolyte layers 1. A plurality of fuel cells C1 to C4 of the same configuration are stacked inside a ceramic vessel 4. The fuel cells are sealed in the vessel 4 by end plates 9, 10 via fillers 7, 8.
The vessel 4 is provided with a feed pipe 5 for a mixed fuel gas including methane or another fuel and oxygen and an exhaust pipe 6 for the exhaust gas. The parts in the vessel 4 other than the fuel cells, that is, the spaces in the vessel 4 through which the mixed fuel gas and exhaust gas flow, are filled by the fillers 7, 8 for suitable separation. Therefore, when driven as a fuel cell configuration, there will no longer be any ignition even if there is mixed fuel gas within the ignition limit.
The fuel cell configuration shown in FIG. 12B is basically configured in the same way as the single chamber type fuel cell configuration shown in FIG. 12A. However, it is structured with the fuel cells including the solid electrolyte layers stacked in the axial direction of the vessel 4 perpendicular to the flow of the mixed fuel gas. In this case, the fuel cells are comprised of porous solid electrolyte layers 1 and porous cathode layers 2 and anode layers 3 formed at the two sides of the solid electrolyte layers 1. A plurality of fuel cells C1 to C5 of the same configuration are stacked in a vessel 4.
On the other hand, the fuel cell configuration explained above was of a type comprised of fuel cells accommodated in a chamber. A system has been proposed arranging a solid electrolyte fuel cell in or near a flame and using the heat of the flame to hold the solid electrolyte fuel cell at its operating temperature so as to generate power (for example, see Japanese Unexamined Patent Publication (Kokai) No. 6-196176). The configuration of this power generation system is shown in FIG. 13.
The fuel cell of the power generation system shown in FIG. 13 is comprised of a tubular body comprised of a zirconia solid electrolyte layer 1, an anode layer 3 comprising a fuel electrode formed at the outside of the tubular body, and a cathode layer 2 comprising an air electrode formed at the inside of the tubular body. The solid electrolyte fuel cell is arranged in a state exposing the anode layer 3 at the part of the reducing flame of the flame f generated from the combustion system 5 supplied with the fuel gas. By arranging it in this way, the radicals etc. present in the reducing flame are used as fuel, the cathode layer 2 inside the tube is supplied with air by convection or diffusion, and power is generated as a fuel cell.
In the single chamber type fuel cell configuration shown in FIGS. 12A and 12B, while not requiring strict separation of the fuel and air like with a solid electrolyte fuel cell configuration of the related art, an air-tight structure has to be adopted. Further, to enable drive under a high temperature, a plurality of sheet shaped solid electrolyte fuel cells were stacked and connected using interconnects having heat resistance and high electrical conductivity so as to raise the electromotive force. Therefore, a single chamber type fuel cell configuration using sheet shaped solid electrolyte fuel cells suffers from the problems of having a bulky structure and rising in cost. Further, at the time of operation of this single chamber type fuel cell configuration, the temperature is gradually raised until a high temperature so as to prevent cracking of the solid electrolyte fuel cells, so the time until startup is long and trouble is involved.
As opposed to this, in the tubular solid electrolyte fuel cell shown in FIG. 13, the flame is directly utilized. This type of fuel cell configuration does not require the solid electrolyte fuel cell to be accommodated in a sealed structure vessel and therefore has the feature of being an open type. Therefore, in this fuel cell configuration, the startup time can be shortened and the structure is simple. Therefore, this can be said to be advantageous for reducing the size, lightening the weight, and reducing the cost of the fuel cell configuration. Further, in the sense of directly using a flame, incorporation into general combustion systems or incineration systems becomes possible and use as a system for supplying power can be expected.
However, in this type of fuel cell configuration, since the anode layer is formed at the outside surface of the tubular solid electrolyte layer, the radicals in the flame cannot be supplied to the top half of the anode layer and therefore the entire surface of the anode layer formed at the outside surface of the tubular solid electrolyte layer cannot be efficiently utilized. Accordingly, the power generation efficiency was low. Further, since the solid electrolyte fuel cell was directly heated by the flame, it was susceptible to cracking and fracturing due to the sharp changes in temperature. The cracked or fractured solid electrolyte fuel cell then ended up breaking apart making generation of power impossible.
Further, if trying to obtain a high electromotive force in a solid electrolyte fuel cell configuration, as shown in FIGS. 12A and 12B, it was necessary to prepare and stack a plurality of fuel cells each comprised of a solid electrolyte layer formed with a cathode layer and anode layer on its two sides. Further, even in the case of a fuel cell comprised of a tubular solid electrolyte layer formed with a cathode layer and an anode layer at its inside surface and outside surface shown in FIG. 13, it is necessary to prepare the number of fuel cells corresponding to the magnitude of the electromotive force required. Therefore, when the output current may be small, but a high electromotive force is required, the configuration ends up becoming bulky and a reduction of size or reduction of cost cannot be achieved.